1. Field of the Invention
The present invention relates to a ceramic composition, and more particularly, to a ceramic composition which can be used as the material for electronic parts.
2. Description of the Related Art
The demands for an increased signal propagation speed, high density, and lower cost of electronic parts, have brought the following essential requirements for the package and the substrate material constituting the electronic parts:
(1) a low dielectric constant; (2) characteristics such as a thermal expansion coefficient matching that of a semiconductor device (low thermal expansion ratio); (3) a high mechanical strength; (4) a high thermal conductivity; and (5) a low cost. In addition, the reliability as an electronic part, such as hermeticity, is also essential.
In response to these requirements, ceramics such as aluminum nitride ceramic, silicon carbide ceramic, low temperature fired ceramic, mullite ceramic, have been developed.
Among these ceramics, mullite ceramic has a lower dielectric constant and lower thermal expansion ratio than those of the alumina ceramic used in the prior art, but has substantially the same material cost. Another ceramic having a low dielectric constant and low thermal expansion ratio is a low temperature fired ceramic, but this has higher material cost and lower strength than the above mullite. Also, although a lower electric resistivity material (gold, silver, etc.) can be used as the metallizing material, compared with the tungsten or molybdenum employed for an alumina ceramic or mullite ceramic, the problem of cost or technique arises. Further, the soldering material is limited for a low temperature fired ceramic.
In contrast mullite ceramic has the advantages that the metallizing or soldering of external leads, and the formation of a resistor by post-firing can be carried out with a high reliability by using the methods applied to a conventional alumina ceramic; that in the constitution of a package, when a metal plate of molybdenum or a metallized high thermal conductivity ceramic is soldered as the heat dissipating portion, both the material and the method which have been conventionally used for alumina ceramic are applicable; and that, different from the case of alumina ceramic, the mullite ceramic guaranties a reliable bonding strength between the ceramic and the heat dissipating plate as well as between the ceramic and the semiconductor device.
Accordingly, mullite ceramic is most suitable for meeting the requirements for a package and substrate material for electronic parts, but has the following disadvantages in coping with recent demands for a higher speed and higher density of the electronic parts.
(1) When mounting a bare chip or flip chip to increase the density, the mullite ceramic of the prior art has an inadequately small thermal expansion coefficient.
(2) To obtain a higher signal propagation speed, the dielectric constant of mullite ceramic of the prior art, i.e., 7.0 or higher, is not sufficiently low. Namely, a value of lower than 7.0 is required as the dielectric constant for the above purpose.
A mullite ceramic with a low dielectric constant, for example, can be obtained by lowering the density of the sintered product, but in this case, other characteristics such as the mechanical strength will be undesirably lowered. Also, as the flexural strength of a mullite ceramic, a value of more than 3000 kg/cm.sup.2 has been obtained by using a very pure and fine starting material powder, but these sintered products, due to minute amounts of liquid phase formed during the firing can form a conductor by the co-firing of conductor materials such as tungsten and molybdenum, whereby the adhesive strength of the metallized layer obtained by firing is too low for practical application. The adhesive strength of the metallized layer can be improved by adding, for example, a group IIa element compound such as MgO to the starting material powder of mullite, but in this case a problem arises in that the flexural strength of the sintered product is very low.